In the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Releases 8 to 11, such modulation modes as Quadrature Phase Shift Keying (QPSK), 16QAM and 64QAM, with modulation orders of 2, 4 and 6, respectively, may be used for the transmission of downlink data. For different channel coding rates, there are 29 different MCSs for the downlink data transmission, which correspond to IMCS 0 to 28 in Table 1, respectively.
TABLE 1MCSModulationTBSIndexOrderIndexIMCSQmITBS0201212223234245256267278289291049114101241113412144131541416415176151861619617206182161922620236212462225623266242762528626292reserved304316
In Table 1, rows corresponding to IMCS 29, 30 and 31 represent the modulation modes, i.e., QPSK, 16QAM and 64QAM, for a retransmitted downlink data packet, respectively. In downlink control information (DCI), 5 bits are used to indicate different MCSs, i.e., IMCS 0 to 31. A second column in Table 1 represents the modulation orders, e.g., the modulation orders for QPSK, 16QAM and 64QAM are 2, 4 and 6, respectively. A third column in Table 1 represents a downlink transport block size (TBS), and different values represent different TBS indices.
It will be considered in the standardization of the 3GPP LTE Release 12 to introduce downlink 256QAM into the Small Cell Enhancement Project, so as to further increase a downlink data rate in the room and at hot regions. However, in the LTE Releases 8 to 11, the DCI can merely be used to indicate QPSK, 16QAM and 64QAM for the downlink data transmission, rather than 256QAM.
Currently, there is an urgent need to introduce 256QAM into the 3GPP LTE Release 12. However, the 3GPP LTE Release 12 is still in a research phase, and no scheme has been proposed for indicating 256QAM for the DCI.